Fish and plants prefer fairly neutral water, with a pH level between 6.8 and 7.2. This a critical part of a healthy aquaponics system, as it has a huge impact on all the living parts of the balance. Low pH will slow down the nitrification process, and can create very uncomfortable conditions for your fish residents. Low pH can be caused by your source water, the materials in your system, or what kind of plants you're growing. Your pH levels can be raised by adding a mixture of calcium and potassium carbonates; gradually changing the water within your system with more neutral water; adding sodium hydroxide; or using alkaline grow mediums, like crushed limestone.
Water with a high pH level can halt the nitrification process completely. This typically occurs when carbonate builds up within your system, due to hard water, or your building materials. This is also commonly seen in young or cycling systems. When first cyling your system, the water's pH is lowered by allowing the bacterial colonies to establish and start producing nitrates. Later on in your system's lifetime, a high pH level can be solved by adding certain acids, like phosphoric acid. Alternatively, some filters, like reverse osmosis filters, can remove carbonates from the water, and assist in lowering pH levels.
